The present invention relates to a recovery boiler plant of a chemical pulp mill having a smelt spout area comprising smelt spouts, which are connected to the lower part of the boiler for discharging smelt from the boiler into a dissolving tank, and a working area in front of the smelt spouts. The invention also relates to a method at a recovery boiler plant of a chemical pulp mill.
An essential apparatus in the recovery cycles of sulfate- and other Na-based pulping processes is the recovery boiler for waste liquor containing cooking chemicals, such as a soda recovery boiler, wherein the chemicals are processed into a form suitable for recovery purposes. The most important chemicals in a sulfate process are sodium and sulfur. Organic substances that have dissolved in waste liquid in the cooking are combusted in the boiler, generating heat that is utilized on one hand for converting inorganic compounds contained in the waste liquid back to chemicals to be used in cooking, and on the other hand for generating steam. The inorganic matter of the waste liquid melts in the high temperature of the boiler and flows in form of smelt onto the bottom of the furnace.
From the bottom of the boiler the chemical smelt is led via cooled smelt spouts into a tank, wherein it is dissolved in water or weak white liquor for producing soda liquor, i.e. green liquor. The main components of smelt, and thus also of green liquor, in the sulfate process are sodium sulfide and sodium carbonate. The green liquor is then led to the causticizing plant, where it is used for producing white liquor.
The dissolving tank is a large cylindrical tank having a horizontal bottom and cover. The tank is usually made of black plate and the interior of it is lined with water-proof concrete or acid-proof plate. The smelt is usually dispersed by directing a steam jet into the smelt stream flowing from the spout. Small explosions happen in the dissolving tank all the time, as anyone who has been near to dissolvers can conclude from the noise. Hot smelt flow causes crashes or explosions when falling into the dissolving tank. The noise is due to explosion reactions between smelt and water as smelt meets green liquor in the dissolving tank. The temperature of the smelt is in the range of 750-820° C. and the temperature of the green liquor (or weak white liquor) containing mainly water in the dissolver is in the range of 70-100° C.
The part of the smelt spout that extends to the outside of the wall of the furnace of the boiler is surrounded usually by a closed hood, i.e. protective housing, by means of which liquid and smelt splashes and vent vapors are prevented from passing to the surroundings. The bottom part of the hood is connected to a smelt dissolving tank located below the smelt spout, into which tank the smelt falls from the spout and wherein the smelt is dissolved in liquid forming green liquor. Nozzles spraying a medium for dispersing the smelt stream are typically mounted in the hood and directed towards the smelt stream falling from the spout. Smelt splashes can enter and stick to the hood and the walls of the dissolving tank. Smelt cakes thus formed also cause explosions when falling into the dissolving tank.
Working conditions of the recovery boiler are demanding. Splashes of hot chemical smelt, liquor, hot surfaces, high ambient temperature, chemical fumes in the breathing air and noise cause hazardous situations. Working in the vicinity of the smelt spouts is inevitable due to control, maintenance and overhaul work. It can e.g. be necessary to rod the smelt spouts, because of possible cloggings, which prevent the smelt from flowing. The noise exposure in chemical pulp mills is often over 85 dB.
FI-patent publication 121313 discloses a solution, where a shielding wall is arranged in front of the smelt spouts so that the operator is safe behind the wall when performing measures in the smelt spout area during operation. Thus, the shielding wall is located between the smelt spouts and the working area.
The heat in the smelt spout area, the noise and gases and smelt splashes possibly entering from the smelt spouts disturb also those who do not actually work in this area but are to walk or work in the vicinity thereof.